High rotational speed vacuum pump

ABSTRACT

A high rotational speed vacuum pump ( 10 ) includes a compressor ( 12 ), a drive motor ( 14 ) for driving said compressor ( 12 ), and a motor control unit ( 16 ) for operating said drive motor ( 14 ) at a constant nominal rotational frequency. Depending on the basic conditions, undesirably strong oscillations may occur at the compressor ( 12 ) during compressor operation at the nominal rotational frequency. These undesirably strong oscillations cause the bearings to be excessively stressed. To reduce the oscillation stress of the compressor and/or the vacuum pump, an activatable rotational frequency correction device ( 18 ) is associated with the motor control unit ( 16 ). The correction device constantly corrects the nominal frequency by a rotational frequency correction value of max. 10% of the nominal rotational frequency when the rotational frequency correction device ( 18 ) is activated. This allows the rotational frequency of the drive motor ( 14 ) to be changed in a simple manner when strong natural oscillations of the compressor ( 12 ) and/or the vacuum pump ( 10 ) occur such that resonance effects and the like can be avoided.

BACKGROUND

The invention relates to high rotational speed vacuum pumps, for exampleturbomolecular pumps.

In the present case, the term high rotational speed vacuum pump relatesto vacuum pumps with nominal speeds as from 10,000 revolutions perminute. High rotational speed vacuum pumps, for example turbolmolecularpumps, are normally operated at a constant nominal rotational speed ornominal rotational frequency. The nominal rotational frequency isdetermined such that the resonant frequency or the resonant frequenciesof the vacuum pump are not excited. While the nominal rotationalfrequency at which the motor control unit operates the drive motorvirtually remains unchanged and constant, the resonant frequencies ofthe vacuum pump may change depending on numerous basic conditions. Theresonant frequency of the vacuum pump is in particular influenced bychanges of the vacuum pump mass, suspension and damping. In plantscomprising a plurality of high rotational speed vacuum pumps which areall mounted on a single frame and are thus mechanically interconnected,the vacuum pump oscillations having the same frequency may combine intooscillations with a large amplitude, and/or the resonant frequency ofone or a plurality of vacuum pumps may change such that the respectivevacuum pump is excited by a vacuum pump operating at the nominalrotational frequency. Studies have shown that wear of vacuum pumpbearings used in this type of plant is to a large extent caused by theeffects described above.

It is an object of the invention to provide a possibility of reducingthe wear caused by natural oscillations of high rotational speed vacuumpumps.

SUMMARY

According to one aspect, an activatable rotational frequency correctiondevice is associated with the motor control unit, said correction deviceconstantly correcting the nominal rotational frequency by a rotationalfrequency correction value of max. 10% of the nominal rotationalfrequency when the rotational frequency correction device is activated.Provision of the rotational frequency correction device allows the perse constant rotational frequency to be slightly changed. In this manner,the rotational frequency of the drive motor can be slightly adjustedpreemptively or according to requirement, which normally is an adequatemeasure for eliminating resonance behavior of the vacuum pump at thenominal rotational frequency, or prevent the generated oscillations fromsumming up and building up when a plurality of vacuum pumps are mountedon a common frame. For this purpose, a rotational frequency correctionvalue of max. 10% of the nominal rotational frequency is completelysufficient. Normally, even a rotational frequency correction value of0.5-2% of the nominal rotational frequency suffices for preventing theoccurrence of resonance and/or a build-up of oscillations. In thismanner, in particular wear of the bearings is reduced.

Preferably, a rotational frequency correction activator is providedwhich may be connected to the rotational frequency correction device foractivating the latter. The rotational frequency correction activator maybe an external electronic storage element, for example, which is adaptedto be electrically connected to the rotational speed correction device.Once the rotational frequency correction activator is connected to therotational speed correction device, the nominal rotational frequency isreduced by a value stored in the rotational speed correction device orin the rotational frequency correction activator.

The external electronic storage element may be configured as a dongle,for example, which is connectable to an electric interface of the motorcontrol unit and/or the rotational frequency correction device.Connecting the dongle to the interface causes the rotational speed to becorrected in a simple manner. Storage elements and/or dongles withdifferent rotational speed correction values may be provided such thatthe value and the sign of adjustment of the nominal rotational frequencycan be selected.

According to a preferred aspect, an oscillation sensor connected to therotational frequency correction device is associated with thecompressor, wherein the rotational frequency correction device isactivated when the oscillation sensor detects that a predeterminedoscillation amplitude of the compressor has been exceeded. Provision ofan oscillation sensor and evaluation of the oscillation signal providedby said oscillation sensor allow the vacuum pump to be permanentlymonitored. Thus the vacuum pump is virtually maintenance-free withregard to prevention of strong natural oscillations. Further, it is thusensured that a rotational frequency correction is performed when theresonant frequency of the vacuum pump changes in a critical manner onlyafter startup of the vacuum pump.

Preferably, at least two different rotational frequency correctionvalues are stored in the rotational frequency correction device. In thismanner, a second rotational frequency correction can be performed, inparticular in plants comprising a plurality of vacuum pumps mounted on acommon frame, if the first rotational frequency correction turns out tobe inadequate.

According to a preferred aspect, a control device is associated with therotational frequency correction device, said control device receivingthe controlled variable from the oscillation sensor. The rotationalfrequency is the manipulated variable. The control device ensures, inparticular in complex systems comprising a plurality of vacuum pumps,that a rotational frequency correction is performed even undertemporarily changing basic conditions, said rotational frequencycorrection ensuring the smallest possible oscillation of the vacuum pumpand/or the compressor.

Generally, the control device may be provided both in the vacuum pumpitself and, in particular in plants comprising a plurality of vacuumpumps, centrally in a control unit common to all vacuum pumps.

A method according to the invention for operating a high rotationalspeed vacuum pump having a compressor, a drive motor for driving thecompressor, and a motor control unit for operating the drive motor at aconstant nominal rotational frequency, comprises the following methodsteps:

-   -   determining the natural oscillations of the compressor,    -   when the natural oscillations of the compressor exceed a        predetermined limit value: changing the rotational frequency by        a value less than 10% of the nominal rotational frequency of the        drive motor,    -   operating the drive motor at the changed rotational frequency.

The method according to the invention allows the natural oscillations ofthe compressor to be automatically monitored and the frequency of theoscillations generated by the drive motor to be corrected, if necessary.

BRIEF DESCRIPTION OF THE DRAWINGS

Two embodiments of the invention will now be described in greater detailwith reference to the drawings which are for illustratrative purposesonly and are not to be taken as limiting the invention.

FIG. 1 shows a first embodiment of a vacuum pump according to theinvention, wherein the rotational frequency correction is activated by adongle, and

FIG. 2 shows a second embodiment of a compressor according to theinvention, wherein the oscillation behavior is monitored by anoscillation sensor.

DETAILED DESCRIPTION

FIGS. 1 and 2 each show a high rotational speed vacuum pump 10;30configured as a turbomolecular pump. The high rotational speed vacuumpump may also be configured as a radial compressor. The vacuum pump10;30 has a nominal rotational speed of 30,000 revolutions/min., forexample, which corresponds to a rotational frequency of 500 Hz. Thevacuum pump 10;30 may be used as a stand-alone unit, but may also formpart of a plant of 100 to 120 vacuum pumps of the same type mounted on acommon frame. Such plants are employed for glass coating purposes, forexample.

The vacuum pump 10;30 comprises a turbomolecular compressor 12 driven bya drive motor 14. The drive motor 14 may be a direct current or analternating current motor.

The drive motor 14 is driven by a motor control unit 16 which suppliescurrent to the motor windings. The motor control unit 16 operates thedrive motor 14 at a constant rotational frequency.

The vacuum pump 10 shown in FIG. 1 further comprises a rotationalfrequency correction device 18 adapted to be activated by a rotationalfrequency correction activator 20. The activated rotational frequencycorrection device 18 causes the motor control unit 16 to adjust thenominal rotational frequency, e.g. to adjust the rotational frequency bya rotational frequency correction value of 10 Hz.

The rotational frequency correction activator 20 is an electronicstorage element configured as a so-called dongle. The dongle isconnected to a parallel or serial interface 22, when necessary, saidinterface 22 being connected to the rotational frequency correctiondevice 18. In the rotational frequency correction activator 20 therotational frequency correction value is electronically stored.

When the rotational frequency correction activator 20 is not connectedto the interface 22, the rotational frequency correction device 18 isnot activated such that the motor control unit 16 operates the drivemotor 14 at the nominal rotational frequency. Once the rotationalfrequency correction activator 20 is connected to the interface 22 ofthe rotational frequency correction device 18, the rotational frequencyis reduced by the stored value.

In the embodiment shown in FIG. 2 of a vacuum pump 30, the oscillationbehavior of the vacuum pump 30 is controlled by controlling therotational frequency of the drive motor 14. For this purpose, anoscillation sensor 36 is fixed to the compressor 12, said sensor 36sensing the frequency and the amplitude of the natural oscillations ofthe compressor 12. The oscillation sensor may also be arranged at ahousing portion of the vacuum pump 30.

The oscillation sensor 36 is connected to the rotational frequencycorrection device 34 which evaluates the oscillation informationsupplied by the oscillation sensor 36 and causes the rotationalfrequency to be corrected when the oscillation sensor 36 detects that apredetermined oscillation amplitude of the compressor 12 has beenexceeded. If the oscillation sensor 36 detects too strong oscillations,the rotational frequency correction device 34 performs an upward ordownward correction of the nominal rotational frequency by a rotationalfrequency correction value of 10 Hz to obtain a new motor rotationalfrequency. If the oscillation sensor 36 detects inadmissibly highoscillations when the drive motor 14 is operated at the new motorrotational frequency, the nominal rotational frequency is changed by asecond rotational frequency correction value, and the drive motor 14 isoperated at the second new motor rotational frequency.

In the rotational frequency correction device a control device may beprovided which receives the controlled variable from the oscillationsensor 36. The motor rotational frequency may also be infinitelyvariable such that the motor rotational frequency always lies as closeto the nominal rotational frequency as possible.

The invention has been described with reference to the preferredembodiments. Modifications and alterations may occur to others uponreading and understanding the preceding detailed description. It isintended that the invention be constructed as including all suchmodifications and alterations insofar as they come within the scope ofthe appended claims or the equivalents thereof.

1. A high rotational speed vacuum pump comprising a compressor, a drivemotor for driving said compressor, a motor control unit for operatingsaid drive motor at a constant nominal rotational frequency, and anactivatable rotational frequency correction device associated with saidmotor control unit, said correction device constantly correcting thenominal rotational frequency by a rotational frequency correction valueof max. 10% of the nominal rotational frequency when said nominalrotational frequency correction device is activated.
 2. The highrotational speed vacuum pump according to claim 1, further including: arotational frequency correction activator which is connectable to therotational frequency correction device for activating the latter.
 3. Thehigh rotational speed vacuum pump according to claim 2, wherein therotational frequency correction activator includes an externalelectronic storage element which is electrically connectable to therotational frequency correction device.
 4. The high rotational speedvacuum pump according to claim 3, wherein the rotational frequencycorrection activator includes a dongle which is connectable to anelectric interface of the rotational frequency correction device.
 5. Thehigh rotational speed vacuum pump according to claim 1, furtherincluding: an oscillation sensor connected with the rotational frequencycorrection device and associated with the compressor, wherein saidrotational frequency correction device is activated when saidoscillation sensor detects that a predetermined oscillation amplitudehas been exceeded.
 6. The high rotational speed vacuum pump according toclaim 5, wherein the rotational frequency correction device stores atleast two different rotational frequency correction values.
 7. The highrotational speed vacuum pump according to claim 5, further including: acontrol device associated with the rotational frequency correctiondevice, said control device controlling the motor rotational frequencyand receiving the controlled variable from the vibration sensor.
 8. Amethod for operating a high rotational speed vacuum pump having acompressor, a drive motor for driving said compressor, and a motorcontrol unit for operating the drive motor at a constant nominalrotational frequency, the method comprising the following method steps:determining natural oscillations of the compressor by means of anoscillation sensor, when the natural oscillations of the compressorexceed a predetermined limit value, changing the rotational frequency ofthe drive motor by a value less than 10% of the nominal rotationalfrequency of the drive motor and, operating the drive motor at thechanged rotational frequency.
 9. A method for operating the high speedrotational vacuum pump of claim 1 comprising: determining naturaloscillations of the compressor by means of an oscillation sensor, whenthe natural oscillations of the compressor exceed a predetermined limitvalue, changing the rotational frequency of the drive motor by a valueless than 10% of the nominal rotational frequency of the drive motorand, operating the drive motor at the changed rotational frequency. 10.A high speed rotational vacuum pump comprising: a turbomolecular device;a drive motor which drives the turbomolecular device; a motor controlunit which controls the drive motor to operate at a constant rotationalfrequency; a rotational frequency correction device which causes themotor control lunit to decrement or increment the constant rotationalfrequency to inhibit the constant rotational frequency fromcorresponding to a resonant frequency of the turbomolecular device. 11.The high speed rotational vacuum pump according to claim 10, furtherincluding: an oscillation sensor which senses oscillations indicative ofoperation approaching a resonant frequency, the rotational frequencycorrection device causing the motor control unit to decrement orincrement the constant rotational frequency in response to sensedoscillations indicative of operation approaching a resonant frequency.12. The high speed rotational vacuum pump according to claim 10, furtherincluding: an electronic storage element which selectively instructs therotational frequency correction device to cause the motor control unitto increment or decrement the constant rotational frequency.